Considering the components produced by high pressure die casting (HPDC) process usually with ultra-large sizes and complex morphologies, high temperature solid solution treatment is not a suitable method to further improve their mechanical properties. In this study, two-stage aging treatment with different pre-aging times was designed and employed to further improve the mechanical properties of HPDC Al8SiMgCuZn alloy. The characteristics of precipitates were evaluated by a transmission electron microscope (TEM), and the precipitation strengthening mechanism was discussed. The results reveal that the strengthening is mainly contributed by the precipitation of β″ phase after two-stage aging, and the number density and size of the precipitates are significantly depended on the pre-aging time. The number density of precipitates is increased with the pre-aging time prolonged from 0 h to 4 h, and then decreases with the further increase of pre-aging time from 4 h to 6 h. The precipitates with the highest density and smallest size are observed after pre-aging for 4 h. After pre-aged at 100 °C for 4 h and then artificial aged at 200 °C for 30 min, the yield strength of 207 MPa, ultimate tensile strength of 325 MPa and elongation of 7.6% are achieved.